1. Field of the Invention
This invention relates to a copper base alloy having high resistances to seizure, wear and the like, suitable for use as a material of a sliding member, and more particularly to such a copper base alloy suitable for use as a material for a sliding bearing used under severe sliding conditions such as a floating bush bearing of a turbocharger used in automobiles.
2. Description of the Related Art
Automobile engines have recently been provided with turbochargers for the purpose of achieving high power. In turbochargers, an exhaust gas is used to rotate a turbine at high speeds so that a compressor is driven. Accordingly, conditions under which a floating bush bearing supporting a turbine shaft is used are very severe or heavy.
For example, a lubricant pump is stopped simultaneously with stop of an engine when the engine is stopped immediately after high speed rotation. Since a floating bush bearing is ceased to be cooled by a lubricant, the floating bush bearing is heated by a high heated turbine casing such that the temperature of the floating bush bearing exceeds 300° C. When the engine is re-started under these conditions, supply of the lubricant is retarded although the turbine immediately reaches 100,000 rpm. Accordingly, the floating bush bearing bears the turbine shaft without supply of the lubricant (a dried-up condition). The floating bush bearing is required to provide a high anti-seizure property, high wear resistance and the like even under the dried-up condition.
JP-A-3-215642 discloses a material for a sliding member meeting the foregoing requirements. The disclosed material consists of, by mass %, 1 to 3.5% Mn, 0.3 to 1.5% Si, 10 to 25% Zn, 5 to 18% Pb, and balance of Cu and unavoidable impurities. The material thus contains Pb dispersed uniformly throughout the structure and is a high strength brass having a microstructure of which matrix is composed of α-single phase. The material disclosed in JP-A-3-215642 will hereinafter be referred to as “first conventional copper base alloy.”
JP-A-9-316570 also discloses another material meeting the foregoing requirements. The disclosed material consists of, by mass %, 0.3 to 5% Mn, 0.3 to 3% Si, 15 to 37% An, 0.3 to 4% Bi, and balance of Cu and unavoidable impurities. An amount of β-phase in the metal structure of the disclosed material is controlled to be not more tha 30%. The material is a silicified manganese high strength brass and has a cold workability. The material disclosed in JP-A-9-316570 will hereinafter be referred to as “second conventional copper base alloy.”
However, the aforesaid first conventional copper base alloy contains Pb although the alloy is superior in the resistances to seizure, wear and the like. Use of Pb should be avoided since Pb is a substance contaminating environment.
Furthermore, the second conventional copper base alloy contains the β-phase in the matrix. The wear resistance can be improved since the β-phase is hard. However, when used as a bearing alloy for bearings used under severe conditions such as the floating bush bearing of the turbocharger, the second conventional copper base alloy has a possibility of seizure particularly in the driedup state. The possibility of seizure has not been overcome yet.